The present invention relates to a protective device for an injection unit of an injection molding machine.
An injection unit of a type involved here is part of an injection molding machine and typically includes a plasticizing cylinder which produces plastic melt for injection via an injection nozzle into a molding tool. When the injection unit is detached from the molding tool, there is a risk that personnel, working in proximity of the injection unit of the injection molding machine, get injured by squirting hot molding compound that may spew uncontrollably out of the injection nozzle because of a gas bubble formation or thermal expansion of the molding compound in the plasticizing cylinder or because of malfunction of the process sequence.
To address this risk of injury, a protective system has been proposed which includes a pneumatically or hydraulically operated protective flap which is disposed immediately in front of the outlet port of the injection nozzle. The protective flap swings upwards into an opening position for clearing the injection nozzle by means of pneumatic or hydraulic actuators through intervention of controlled proximity switches, as soon as the injection unit has reached during advancement a defined position at the molding tool. Conversely, the flap can be moved into a shielding position for covering the outlet port of the injection nozzle as soon as the injection unit leaves the defined position during a return stroke. This type of protective system is complex in structure and is prone to malfunction because of the series connection of different system components such as an electric proximity circuit, hydraulic or pneumatic actuators, and a mechanical flap kinematics.
Japanese patent publication JP-A-08 025 415 discloses an injection unit with a protective device having a spring-loaded nozzle tip which includes a nozzle channel and is coupled with a shut-off valve via an operating stem extending through the nozzle channel. The shut-off valve is disposed on the inlet side of the nozzle channel and loaded by a spring force to seek the closed disposition. When impacting the funnel-shaped gate of the molding tool, the nozzle tip is pushed back in opposition to the spring force to operate the shut-off valve. This type of protective device suffers many process-based shortcomings. Numerous molding compounds trap gas in the plasticizing cylinder as a result of degradation to develop gas bubbles which are unable to escape to the outside, when the injection unit is uncoupled, because the shut-off valve is closed. Thus, these gas bubbles migrate during the injection stage together with the molding compound into the molding tool, adversely affecting the quality of the injection-molded article or even leading to a rejection of the injection-molded article. There is a further problem of this protective device relating to the closure of the nozzle channel on the inlet side. After injection of a shot, remaining molding compound in the nozzle channel cannot be removed by molding compound subsequently pressed from the plasticizing cylinder because the shut-off valve has assumed the closed position. Thus, remaining molding compound may solidify and ultimately form a plug closure which can migrate into the mold cavity during the next shot so that the quality of the injection molded article is again adversely affected or a malfunction of the valve operation may even be experienced.
It would therefore be desirable and advantageous to provide an improved protective device which obviates prior art shortcomings and which does neither adversely affect the injection molding process nor the nozzle channel geometry.